Precise fuel mixture for a motor (7)
I believe the answer is:
careful
'precise' is the definition.
(thesaurus)
'fuel mixture for a motor' is the wordplay.
'mixture' indicates an anagram.
'for' means one lot of letters go next to another (I've seen this in other clues).
'a motor' becomes 'car' (I've seen this before).
'fuel' is an anagram of 'eful'.
'eful' put after 'car' is 'CAREFUL'.
(Other definitions for careful that I've seen before include "Thrifty" , "Tight-fisted" , "Diligent" , "Attentive" , "'Prudent, cautious (7)'" .)